北米での医学会参加のため搭乗していた旅客機内でドクターコールがあり対応した。目的地の空港のスタッフに情報提供した方が良いと判断し，乗務員に伝えたところ，「所見をメモして欲しい」と依頼され記載した文面を示す。

A 78-year-old female passenger has developed swelling of her left lower leg towards the end of a long-haul flight. She does not complain of any pain at rest. She has pitting edema of her left lower leg, but no color or temperature changes are observed. Calf pain is induced on dorsiflexion of her left foot. Because she suffers from shortness of breath, the possibility of pulmonary embolism should be considered, and transfer to an appropriate hospital is advised.

　原因として考えられるのはどれか。